Hidden Diversity in European Allocreadium spp. (Trematoda, Allocreadiidae) and the Discovery of the Adult Stage of Cercariaeum crassum Wesenberg-Lund, 1934

Abstract

DNA sequences for adult and larval Allocreadium spp. from their natural fish and molluscan hosts were generated. Phylogenetic analyses based on two molecular markers (ITS2 and 28S rDNA) yielded unexpected results regarding the diversity and life cycles of European species. It was found that specimens morphologically consistent with the concept of Allocreadium isoporum (Looss 1894) form two different species-level genetic lineages. For now, the morphological differences between the specimens belonging to different genetic lineages are not discernible; they can infect the same fish species at the same or different localities. However, the species differ in their life-cycle patterns, specifically in terms of larval stages and first intermediate host specificity. Based on molecular markers, the tailed ophthalmoxiphidiocercaria developing in Pisidium spp. was associated with a sexual adult A. isoporum from Alburnus alburnus, Barbatula barbatula and Rutilus rutilus. Representatives of another genetic lineage, recovered from R. rutilus and Scardinius erythrophthalmus, turned out to be conspecific with the enigmatic European larval trematode Cercariaeum crassum Wesenberg-Lund, 1934, from the sphaeriid bivalve Pisidium amnicum. This finding requires the recognition of the cryptic species Allocreadium crassum.